1. Field of the Invention
The present invention relates generally to fixed cutter or drag type bits for drilling subterranean formations and, more specifically, to drag bits for drilling relatively hard, abrasive, or hard and abrasive rock formations.
2. State of the Art
So-called “impregnated” drag bits may be used conventionally for drilling relatively hard, abrasive, or hard and abrasive rock formations, such as sandstones. Impregnated drag bits may typically employ a cutting face composed of diamond impregnated matrix, which may comprise superabrasive cutting particles, such as natural or synthetic diamond grit, dispersed within a matrix of wear resistant material. For example, the wear resistant matrix may typically comprise a tungsten carbide powder infiltrated with a copper-based binder. Thus, the blades or bit face itself may comprise diamond particles which are used to engage the formation and thus drill thereinto. Accordingly, during use of an impregnated drag bit, the embedded diamond particles and the matrix material in which they are dispersed may wear and as worn cutting particles are lost, and new cutting particles may be exposed. Of course, impregnated drag bits may include different types of diamond material, such as natural diamonds, synthetic diamond, and thermally stable diamond material.
Similarly, so-called BALLASET® drag bits employ a cutting face primarily composed of synthetic thermally stable diamond cutting structures which protrude from the matrix material in which they are disposed. Thermally stable diamond, as known in the art, generally comprises polycrystalline diamond sintered material that initially contains a catalyzing material, such as cobalt, which is later removed, as by an acid leaching process. Removal of the catalyst is believed to reduce “back conversion” of sintered polycrystalline diamond to graphite by dissolution within the catalyst at elevated temperatures. Since BALLASET® drag bits may employ diamond cutting structures that extend from the surface of the blades or profile, during use of a BALLASET® drag bit, abrasive wear may occur upon the thermally stable cutting structures.
Conventionally, impregnated or BALLASET® drag bits may be fabricated by similar processes. Particularly, a mold is machined and prepared, often at least partially by hand, to form a shape that is complementary to the shape of the desired drag bit geometry. Diamond cutting structures may be placed within the mold or upon a surface thereof and may comprise natural, synthetic, or thermally stable diamond material. Further, as known in the art, displacements, which may comprise resin-coated sand or graphite, may be formed by machining, grinding, or as otherwise known in the art and placed into the mold to form junk slots, fluid communication ports, or other topographical features of the rotary drag bit. The mold may be filled with a powder or particulate which is preferably erosion or abrasion resistant, such as, for instance, tungsten carbide or an equivalent material. A steel support structure, known as a “blank” in the art, may be disposed at least partially within the mold prior to filling with powder or particulate. The mold may then be placed in a furnace where a suitable copper-based binder or other metal alloy binder is melted and infiltrated into the particulate, so as to form, upon cooling, a body of solid infiltrated matrix material in a complementary shape of the mold, and having thermally stable or natural diamond particles embedded in its outer surface. The blank may also be affixed within the hardened infiltrant and may be sized and configured for post-furnacing machining so as to attach the blank to a hardened, threaded, steel shank, as by welding. This method of construction of infiltrated drag bits is well known in the art.
Alternatively, in the case of an impregnated drag bit, a cutting structure including diamond may be preformed, such as a segment or post, by hot isostatic pressure infiltration or other infiltration process, and subsequently attached to the drag bit body by brazing. In a further alternative method of manufacture, preformed cutting structures may be placed within a mold and affixed to the drag bit by an infiltration process, such as the one described above.
It is well known in the art that rotary drag bits may include a so-called inverted cone region, which refers generally to an indentation formed in the face of the rotary drag bit proximate the longitudinal axis thereof in a direction generally opposing the direction of drilling. It is also known in the art that the drilling fluid ports may extend through the interior of the body of the drag bit and exit the surface of the face of a drag bit proximate the longitudinal axis, within the cone region or as otherwise desired.
Regarding the inverted cone region, conventional approaches to manufacturing usually include forming a cone displacement of a complementary geometry in relation to the desired geometry of at least a portion of the inverted cone region of the rotary drag bit and placing the cone displacement within a mold. A conventional cone displacement will typically comprise a substantially conical body and include recesses that follow relatively straight radial paths, the paths specifically configured for placement of cutting structures, such as natural diamonds or synthetic diamond material, the diamond material to become imbedded within the inverted cone region upon infiltration. Thus, the cone displacement may be positioned substantially centrally at the longitudinal bottom of a rotary drag bit mold and cutting structures, such as natural diamonds or thermally stable diamonds, may be placed upon the surface of the displacement. As a further consideration, a fluid bore displacement, typically comprising resin-coated sand, may mate to the cone displacement along at least a portion thereof to form one or more fluid ports exiting to the face or surface of the rotary drag bit within the inverted cone region.
As may be appreciated, it is desirable that the circumferential position of the radially extending recesses, which are configured for placement of cutting structures, such as diamonds, of the cone displacement are preferably configured so as to not intersect with the mating regions of the fluid bore displacement, because such interference may require that the diamond cutting structures be repositioned. For instance, if the recesses do overlap with mating regions of the fluid bore displacement, modifications may be required to ensure a desired amount of diamond cutting material is included within a central region of the rotary drag bit. Such modifications may be undesirably inconvenient and costly.
In one example of a conventional rotary drag bit design, U.S. Pat. No. 3,599,736 to Thompson discloses a rotary drag bit including a plurality of abrasive particles dispersed along generally radially extending blades. In addition, the rotary drag bit includes an inverted cone region having a surface from which drilling fluid apertures exit. However, as shown in FIG. 2 of U.S. Pat. No. 3,599,736 to Thompson, the intersection of fluid ports 17 with the cutting structures 27 disposed on lands 18 (blades) may require a customized and relatively complicated cone displacement or mold and a mating fluid bore displacement, both of which may be dependent on one another and, therefore, difficult to modify or adapt to different sizes or configurations.
Another conventional rotary drag bit is disclosed in U.S. Pat. No. 2,838,284 to Austin, which includes lands 20 (blades) that spiral generally from the longitudinal axis thereof. Diamond cutting elements 16 are disposed on the lands 20. Also, U.S. Pat. No. 4,550,790 to Link discloses a rotary drag bit having spiral lands 40. Further, U.S. Pat. No. 4,176,723 to Arceneaux discloses a diamond drag bit wherein the diamonds are arranged in a plurality of individual rows, wherein each row extends along a slight spiral from the gage radially inwardly toward the center of the drag bit. Finally, U.S. Pat. No. 3,951,220 to Phillips, Jr. discloses a drag bit which includes an eccentric fluid port and spiral blades that carry carbide buttons.
Since molds used to fabricate rotary drag bits are time consuming and labor intensive to fabricate, improved methods of manufacture may be desired which afford greater flexibility in manufacturing. Although the present invention may be particularly applicable to impregnated drag bits, it may also be applicable to rotary drag bits, including larger natural or synthetic cutting structures that are set in the outer surface thereof, such as BALLASET® drag bits or polycrystalline diamond compact (PDC) drag bits. Thus, it would be desirable for a rotary drag bit to include an inverted cone region that is simplified from a manufacturing standpoint. Also, it would be desirable for a rotary drag bit to include improved drilling structures.